Heat treatment of stainless steel castings



ilnited grates Patent 3,065,119 HEAT TREATMENT OF STAINLESS STEEL CASTTNGS Frank C. Brantlgain, Bryn Mawr, Pa, assignor to The Sharples Corporation, a corporation of Delaware N0 Drawing. Filed Sept. 3, 1959, Ser. No. 837,763 1 Claim. (Cl. 148-135) Percent carbon .040 maximum. Percent manganese 1.00 maximum. Percent silicon 1.00 maximum. Percent phosphorus .040maximum. Percent sulfur .040 maximum. Percent chromium 25.0 to 27.0. Percent nickel 4.75 to 6.00. Percent molybdenum 1.75 to 2.25. Percent copper 2.75 to 3.25. Percent iron Balance.

It is common practice in the processing of a stainless steel casting having a composition of the above character to permit the casting, after being formed, to cool to atmospheric temperature, and then to subject it to three steps for purposes of improving tensile strength, yield strength and percentage elongation as follows:

The first step is to place the casting in an oven heated, for example, to between 1950 F. to 2050 F. to accomplish what is called solution annealing. A two hour treatment is typical.

The next step is to quench the casting, while at an above-mentioned temperature, in a liquid, such as oil or water, having a temperature at or near atmospheric temperature.

Following the quenching the casting is placed in an oven and heated, for example, to between 850 to 950 F. for purposes of aging. A typical aging time is from 2 to 3 hours. It is during the aging step that tensile strength, yield strength and/or hardness are adjusted to desired levels within limitations imposed by composition and prior treatment. Speaking generally, an increase in hardness results in a decrease in ductility, and vice versa. Percentage elongation is a measure of ductility.

Thereafter, the casting is permitted to air-cool to atmospheric temperature.

The foregoing treatment has been found to be relatively satisfactory for small castings having a maximum section thickness of less than 1 inch when an increase in hardness is the only desired result. However, when high tensile strength, yield strength and percentage elongation are required in castings, particularly with section thicknesses approaching and greater than 1, the results of the foregoing treatment become erratic. With castings having section thicknesses greater than 1", the foregoing treatment becomes so erratic and independable as to be virtually totally unsuitable for the purpose.

One evidence of the unsuitability of such treatment, particularly for castings having maximum section thicknesses greater than 1", is that the castings tend to crack during the treatment. Other deficiencies for certain applibetween 1700 and l800 cations are insuflicient tensile strength, insuflicient yield strength, and insufiicient ductility. As an example, heat treated stainless steel castingsfor use in the manufacture of high-speed centrifuge bowls should have a tensile strength of at least 125,000 pounds per square inch, a yield strength of at least 90,000 pounds per square inch, and a percentage elongation of at least 18% in 2 inches. Obviously, they should be uncracked.

Two typical castings having maximum wall thicknesses greater than ll' were treated in accordance with the prior art method described above, and were found to have tensile strengths of 107,500 psi. and 110,000 p.s,i., respectively, yield strengths of 91,000 psi. and 78,800 p,s.i., respectively, percentages of elongation of 6 and 29%, respectively, and both castings cracked during the quenching. The castings were unfinished centrifuge bowls having a maximum thickness, i.e a thickness in the thickest portion, of approximately 3", with sections having thicknesses of approximately 1". The areas having about 1" thicknesses cracked the same as those having greater thicknesses.

The present invention pertains to the treatment of stainless steel castings having compositions of the chat-1 acter set forth above whereby uncracked castings are obtained. In the preferred embodiment such castings are obtained with unusually high tensile strengths, yield strengths and percentage elongations. As an example, an unfinished centrifuge bowl of the same size, shape and composition as those of the preceding paragraph was treated in accordance with the preferred embodiment of my invention, and was obtained in uncracked state with a tensile strength of 136,500 p.s.i., a yield strength of 94,000 p.s.i., and a percentage elongation of 24%.

While my process is particularly applicable to the treat ment of castings having a maximum wall thickness of at least 1", especially in the avoidance of cracking, it is applicable to the treatment of castings of the specified compositions of any size or thickness with more consistent final results. By the term a casting having a maximum thickness of at least 1" is meant a casting any part of which has a thickness of at least 1, irrespective of the configuration or shape of the casting, or the thickness of other parts thereof.

The casting to be treated in accordance with my invention may be prepared in any desired manner, elgfby pouring into a static mold, or by a centrifugal process.

In the practice of my process the casting is first sub. jected to a solution annealing step, e.g. in an oven or furnace, under temperature conditions between 1950 and 2200 F., and particularly between 2000 and 2100" F. in the preferred embodiment. Also in the preferred embodiment such solution annealing step is carried out for a minimum time of 2 hours plus 2 hours per inch of thick ness of the portion of maximum thickness of the casting. More particularly in the preferred embodiment I find an initial time of 2 hours plus an additional time of from 2 t0 3 our per inch of thicknes of the po tion of maximum thickness to be highly satisfactory. It is, of course, underst od that fractional times are applicable to fractional thicknsses, For example, a casting having a maximum, thickness of 3" would require a minimum treatment time of 8 hours, and a casting having a maximum thickness of 3. /2", a minimum treatment time of 9 hours. v

The next step is to reduce the temperature of the casting to between 1200 and 1800 F., and particularly to F. in the preferred embodiment, e.g. in an oven or furnace, and to hold the casting under such temperature conditions for a sufi icient lengthv of time to assure that all parts of the casting reach such temperature level or range. This step may be carried out in the same oven or furnace or other device employed for the first step, or the casting may be transferred to another oven or furnace or other device, or this temperature reduction may be carried out in other manner. The rate at which this reduction in temperature is effected is not found to be critical, but preferably an extraordinarily wide differential in temperature between portions of the casting during this cooling is to be avoided, such as would be occasioned by a partial quench.

After the preceding stabilization treatment, the casting is quenched in a liquid, e.g. agitated oil or water, down to a temperature below say 150 F e.g. down to ambient temperatures.

Optionally the casting may thereafter be placed in an oven or furnace in which it is held at a temperature of from say 500 to 600 F. for at least one hour for stress relief purposes. I find a treatment time of from 1 to 2 hours to be highly satisfactory. The casting is then permitted to cool, if the final age-hardening step is not to follow without intermediate cooling, depending upon shop routine. Preferably the cooling takes place rather slowly, e.g. by contact with the atmosphere, cooling in still air being ideal.

As has been indicated, the step of stress relieving may be omitted, if desired. This is particularly true if the agehardening treatment is to be performed directly after quenching, e.g. within say several hours, typically within 6 hours. If the casting is to be cooled and stored, say for more than a day, the stress relieving treatment is recommended, though not essential.

In any event, the casting is age-hardened, for which purpose it is heated in an oven or furnace or other device to a temperature between say 800 and 1050 F depending upon the final properties, such as of hardness and ductility, that may be desired, as will be understood by the skilled in the art, and particularly between 925 F. and 975 F., in the preferred embodiment for the properties of unusually high tensile strength, yield strength, and percentage elongation. In the preferred embodiment the casting is held under such temperature conditions for at least three hours, e.g. for from between 3 to 5 hours, of which 4 hours is typical.

Thereafter the casting is permitted to cool, preferably slowly, e.g. by contact with atmospheric air, cooling in still air being ideal.

It is found that the minimum treatment times for solution annealing, as well as the elevated temperature ranges for stabilization prior to quenching, given above, are individually critical in obtaining desired properties in the final age-hardened casting, and are best used in combination in preparation of a casting for age-hardening.

The following examples are given by way of illustration and not by limitation.

Example 1 The casting treated was an unfinished centrifuge bowl having a maximum wall thickness of about 3" with sections of about 1" thickness. The metal had an analysis After casting, the centrifuge bowl was subjected to annealing at a temperature of 2050 F. for a period of ten hours. It was then cooled to a temperature of 1700 F. and held at this temperature for a sufiicient length of time to assure thermal equilibrium throughout the casting. The casting was then quenched in oil to a temperature below 150 F. Thereupon the casting was subjected to the step of stress-relieving at a temperature of 600 F. for a period of one and three-quarter hours. The casting was then permitted to cool slowly by contact with atmospheric air. After cooling to below 150 F. the casting was age-hardened for a period of four hours at a temperature of 975 F. whereupon it was permitted to cool slowly by contact with atmospheric air.

The casting was then subjected to test and was found to have a tensile strength of 136,500 p.s.i., a yield strength of 94,000 p.s.i. and a percent elongation of 24%. There was no evidence whatsoever of cracking.

Example 2 An unfinished centrifuge bowl of the same size and shape and of the same material as the bowl employed in Example 1 was subjected to an annealing temperature of 2050 F. for a period of 4 hours. It was then cooled to 1800 F. and held at this temperature until thermal equilibrium was reached. This was followed by oil quenching to a temperature below 150 F. Thereupon the casting was subjected directly to an aging treatment at a temperature of 950 F. for a period of one hour, and thereafter permitted to slowly cool to atmospheric temperature. The casting didnot crack.

Tests showed that the tensile strength was 110,000 p.s.i. and the yield strength was 78,800 p.s.i. The percent elongation was 29%.

The reduction in temperature to 1800 F. prior to quenching as taught by this invention avoided cracking. The shorter solution annealing time was in very large part responsible for lower resulting tensile strength and yield strength which demonstrates the value of longer annealing times as taught herein.

Example 3 A centrifuge bowl of the type treated in Example 1 was subjected to a treatment similar to that set forth in Example 1, except that the temperature was reduced to 1800 F. instead of 1700 F. prior to quenching, and the step of stress relieving was omitted. Age hardening at 950 F. followed immediately after quenching. The tensile strength was 139,500 p.s.i., the yield strength 111,000 p.s.i., and the percentage elongation was 20%. There was no cracking.

Example 4 A centrifuge bowl of the type treated in Example 1 was subjected to solution annealing at 2050 F. The temperautre was then reduced to only 1950 F. prior to quenching. The bowl cracked.

Example 5 Four stainless steel samples of approximately 2" in maximum thickness and falling within the range in composition set forth above were solution annealed at 2050 F. for 2 hours and then reduced in temperature to 1600 F., 1500 F., 1400 F. and 1200 F., respectively, prior to quenching. The tensile strengths after age-hardening were 109,600 p.s.i., 99,900 p.s.i., 94,500 p.s.i. and 93,500

p.s.i., respectively. The yield strengths were 88,000 p.s.i., 78,900 p.s.i., 76,200 p.s.i. and 78,600 p.s.i., respectively. The percentage elongations were 25.5%, 26.0%, 15.0% and 10.5%, respectively. There Was no cracking in any instance.

Example 6 Three centrifuge bowls of different composition from that of Example 3 and from each other but falling within the range of composition set forth above were treated similarly to the centrifuge bowl of Example 3. The tensile strengths were 135,000 p.s.i., 138,000 p.s.i., and. 138,500 p.s.i., respectively. The yield strengths were 96,000 p.s.i., 96,000 p.s.i. and 94,000 p.s.i., respectively, The percentage elongations were 22%, 20%, and 20%, respectively. There was no cracking.

The foregoing examples amply demonstrate the out standing results obtained by the practice of the invention with compositions of the character set forth at the beginning of this specification, and while exact analytical percentages have been given for such compositions, it will be understood by persons skilled in the art that some variation is possible without significant change in properties. Therefore, the term approximate compositionas used in the claims is intended to encompass not only such compositions but also variations thereof having essentially the same properties.

Moreover, having particularly described my invention, it is to be understood that this is by way of illustration, and that changes, omissions, additions, substitutions and/ or other modifications may be made without departing from the spirit thereof. Accordingly, it is intended that the patent shall cover, by suitable expression in the claim, the various features of patentable novelty that reside in the invention.

1 claim:

A centrifuge bowl made by subjecting a casting to solution annealing under temperature conditions between approximately 1950 P. and 2200 F. for a minimum time of two hours plus two hours per inch of thickness of the thickest part of said casting, thereafter stabilizing said casting by reducing its temperature to between approximately 1200 F. and 1800 F., then quenching said casting, and thereafter subjecting said casting to age-hardening, said casting having an approximate composition as follows:

Percent carbon .040 maximum. Percent manganese l.00 maximum. Percent silicon L00 maximum. Percent phosphorus .040 maximum. Percent sulfur .040 maximum. Percent chromium 25.0 to 27.0. Percent nickel 4.75 to 6.00. Percent molybdenum 1.75 to 2.25. Percent copper 2.75 to 3.25. Percent iron Balance.

References Cited in the file of this patent UNITED STATES PATENTS 2,254,959 Gorrnan Sept. 2, 1941 2,686,116 Schempp Aug. 10, 1954 2,715,576 Payson et a1 Aug. 16, 1955 2,826,496 Kegerise Mar. ll, 1958 2,903,386 Waxweiler Sept. 8, 1959 

